1. Field of the Invention
The present invention relates to a self-drilling and thread-forming connecting element, in particular a screw, comprising a threaded shank which consists of a cylindrical core and of a thread formed as a protuberance on the core, and comprising an end drilling tip for drilling a core hole for the thread.
2. Description of Related Art
Connecting elements which can be screwed into relatively hard materials, such as, for example, hard fiber materials, but also metals (sheets), while automatically forming a thread are known. Sheet-metal screws, for example, may be mentioned here. For this purpose, however, before the screw is screwed in, a core hole must be pre-drilled in a separate operation, the diameter of the core hole being dimensioned to be appropriately smaller in adaptation to the design of the thread on the threaded shank, so that a thread can be formed automatically in the core hole.
German Utility Model 297 11 128 discloses a connecting element of the generic type. In order to avoid the awkward and time-consuming pre-drilling of a core hole, the connecting element of the generic type has an end drilling tip, so that, directly during the screw-in operation, in the end region of the drilling tip, a core hole having a diameter corresponding to the core of the threaded shank is drilled in the respective material by chip removal. The threaded shank adjoining the drilling tip is subsequently screwed into this core hole like a sheet-metal screw. In the process, the corresponding internal thread is formed automatically. The known connecting element has already proved to be very successful in practical use. It is conceived in particular for hard fiber materials. In this case, however, a problem has been found in practice, to be precise specifically with xe2x80x9cMDF boardsxe2x80x9d (MDF=medium density fiber board), to be precise if the screw is to be screwed into the board at the end face, that is from the cut edge. This is the case, for example, for the fastening of fittings, corner screwed connections, etc. In this application, when the respective screw is being screwed in, splintering or bursting of the fiber board often occurs, which then jeopardizes the fastening function.
The object of the present invention is therefore to provide a connecting element of the generic type which is also suitable for this special application without any problems, that is to say also for screwed connections at the end face in MDF boards.
According to the invention, this is achieved in that the drilling tip defines a core-hole drilling diameter which is larger than the diameter of the cylindrical core of the threaded shank and smaller than a thread diameter defined by an outer thread edge of the threaded shank.
By this measure according to the invention, when the screw is drilling in and being screwed in, a clearance space of annular cross section is formed inside the material directly behind the drilling tip, and this clearance space serves to receive the cuttings produced during the drilling operation. The invention is thus based on the knowledge that, when the known connecting elements are being used, the cuttings produced accumulate very quickly and produce an increase in pressure, which then causes the splintering of the material. By means of the invention, the cuttings produced can be distributed in the clearance space behind the drilling tip in such a way that they remain there, essentially free of pressure, between the thread turns when the screw is screwed in further. By the drilling diameter being smaller than the outer thread diameter, reliable retention of the connecting element in the respective material is ensured. For this purpose, it is especially expedient if the drilling diameter is about 20 to 30% larger than the core diameter and about 20 to 25% smaller than the thread diameter.
The connecting element according to the invention may preferably have two alternative embodiments. In the first embodiment, the drilling tip has a relatively short length which is at most approximately equal to the core diameter. The cuttings can pass across this short path very quickly and virtually without hindrance into the clearance space in the region of the threaded shank between the thread turns and the core hole. In this case, the drilling tip preferably has two diametrically opposite cutting edges starting from a spot-drilling point and defining a cutting angle, a flute extending approximately axially in the direction of the threaded shank being arranged upstream of each cutting edge as viewed in the screw-in direction. These flutes form passages via which the cuttings can pass to the rear from the drilling region into the clearance space.
The second advantageous embodiment has a relatively long drilling tip, the cutting angle of which is preferably less than 90xc2x0, in particular 40xc2x0 to 60xc2x0. In contrast to the first embodiment, a thread-free section preferably adjoins the drilling tip. This embodiment according to the invention ensures that, in particular in thin boards of about 12-15 mm, in the case of a screwed connection at the end face and an edge screwed connection, the wall of the drill hole cannot break away or become warped, since the cuttings which occur are effectively discharged.
Further advantageous design features of the invention are contained in the subclaims and the following description.